The concept of filling the cells of honeycomb with various substances is well-known. Among the materials used as fillers are inorganic aggregate particles coated with foaming phenol resin (Jap. Kokai No. 62-181,133); low-density phenolic resin foam (Brit. No. 2,174,335); foam comprising phenol resin and epoxy or polyurethane resin (Jap. Kokai No. 60-72723); finely divided insulating material, e.g., microporous material such as silica gel, volatized SiO.sub.2, CaSiO3, vermiculite, perlite, alumina or titania (U.S. Pat. No. 4,564,547); incombustible filler having a density below 0.2 kg/cu.dm. (E. Ger. No. 211,757), foamed or foamable inorganic grains (Jap. Kokoku 84-31455); urethane foam (Jap. Kokai No. 59-33135); powdered brazing metal and abradable material both mixed with a binder and plasticizer (U.S. Pat. No. 4,546,047 and 4,409,054); cement (U.S. Pat. No. 4,403,008); foamed urea resin (Jap. Kokai No. 57-135143), intumescent insulating material, e.g., epoxy or vinyl or silicone resin, sodium silicate, silicone rubber, butyl rubber, magnesium oxide or magnesium chloride preferably together with various fillers (U.S. 4,299,872 ); sintered metal powder paste (Jap. Kokai No. 56-77302); thermoplastics (U.S. Pat. No. 4,265,688); carbon microballoons (U.S. Pat. No. 4,193,829 and U.S. Pat. No. 4,135,019); foamed resin (U.S. Pat. No. 4,330,494); curable slurries of inorganic materials (Jap. Kokai No. 54-13530); cellular material (Fr. No. 2,356,511); filler coated foam resin liquid (Jap. Kokai 53-8666); foamable inorganic material (Jap. Kokoku No. 84-31454); fire-extinguishing agents of liquid, gaseous or powdery type (U.S. Pat. No. 4,252,579); foam material (U.S. Pat. No. 4,054,477); foam resin (U.S. Pat. No. 4,045,267); silicone resin (U.S. Pat. No. 3,956,050); porous nickel-aluminum alloy (Fr. No. 2,258,524); granular or pelleted non-combustible carrier material coated with an intumescent fire resisting paint or compound (Brit. No. 1,387,582); fibers (Brit. No. 1,412,926); thixotropic material, e.g., a mixture of liquid synthetic resin and finely powdered silicon oxide (U.S. Pat. No. 3,886,023); foamed polystyrene granules (U.S. Pat. No. 3,650,871); polybenzimidazole resin mixed with aluminum powder, glass fibers colloidal silica or hollow glass microspheres (U.S. Pat. No. 3,549,468); and so on. Some additional prior efforts associated with filling the cells of cellular structures such as honeycomb with foams or fibrous materials are referred to, for example, in U.S. Pat. Nos. 3,578,526; 3,630,813; 3,733,229; and British No. 1,331,266.
In a NASA report entitled Development of Fire-Resistant U.S. Pat. No. Low Smoke Generating. Thermally Stable End Items for Aircraft and Spacecraft (NAS9-15050, June, 1977, page 24) a method for filling honeycomb cells with polyimide foam is described. In this method the honeycomb panel was first dipped into a polyimide precursor resin solution several times to fully coat the cell surface area. The panel was then dried in a vertical position to allow excess resin to collect in the bottom of the cells. Drying was conducted in a circulating air oven for 14 to 16 hours at 82-93C. The panels were then foamed at 287.7-316.degree. C. for 15 to 30 minutes and during the heating the resin foamed up to completely fill the cells. The panel was then surface finished to provide a level plane between the foam and honeycomb for receiving panel skins to be bonded thereon. In a subsequent report entitled Development of Fire-Resistant, Low Smoke Generating, Thermally Stable End Items for Commercial Aircraft and Spacecraft Using a Basic Polyimide Resin (NAS9-15484, covering work in the period of December 1977 to April 1980, pages 129-130) the same general method was used and optimized and involved dipping, drying, foaming, curing, and trimming excess foam to flush with the panel surface. This same report describes a modification of this process in which the polyimide precursor resin solution was spray coated onto a preheated honeycomb thereby eliminating the need for the drying step.